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Abstract

Background. Small conductance Ca2+-activated K+ (SK) channels play significant roles in regulating the excitability of cardiomyocytes (CMs). SK channels are unique in that they are gated solely by intracellular Ca2+ and hence, function to integrate intracellular Ca2+ and membrane potentials on a beat-to-beat basis in the heart. Our previous studies revealed that cardiac SK2 channels coupled with L-type Ca2+ channels (LTCCs) through a physical bridge, α-actinin2, suggesting that LTCCs may be functionally coupled with SK2 channels by providing local Ca2+ domain to activate the SK channels. However, a recent study suggested that sarcoplasmic reticulum (SR) Ca2+ release is necessary and sufficient for the activation of cardiac SK channels. The objective of the study is to examine the mechanisms of SK channel activation in native CMs.

Methods and Results. By using a voltage-clamp protocol in rabbit CMs to activate LTCCs followed immediately by a test voltage to monitor the SK currents, we recorded apamin-sensitive SK currents in response to the Ca2+ influx. By altering the duration of activation of LTCCs, the activation of SK currents as a function of the increasing Ca2+ influx was quantified. EGTA or BAPTA were then used to test the intracellular Ca2+ domain necessary for SK channel activation. BAPTA but not EGTA completely disengaged the coupling between LTCCs and SK channels. SK currents coupled with Ca2+ influx via LTCCs continued to be elicited after application of caffeine, ryanodine or thapsigargin to deplete SR Ca2+ store, suggesting that LTCCs provide the immediate Ca2+ microdomain for the activation of SK channels in CMs. Super-resolution imaging of SK2, Cav1.2, and ryanodine receptor 2 (RyR2) using stimulated emission depletion (STED) microscopy was performed to directly quantify the distance among SK, LTCC and RyR2 in CMs.